Paul makes an interesting point, too, in Romans 9 where he says that not all who descend from Israel area really Israel. And he then quotes Isaiah who said that not all Israel will be saved, but only a remnant.
Romans 11, to me, is pretty powerful, also. The REASON the Gentiles were selected is NOT because Israel was rejected. No, it was so that Israel, as a nation, would be incited to jealousy over God's choice to allow the Gentiles in. The whole discussion of the branches of the olive tree is in this chapter, and it's clear that if Israel starts to have faith once again, they will be grafted back in.
As you pointed out, thought, this would occur after the full number of the Gentiles (or fullness of the Gentiles) are brought in.
There's another reason why the GB (and others like them) may not be in the new covenant.
33 “For this is the covenant that I will make with the house of Israel after those days,” declares Jehovah. “I will put my law within them, and in their heart I will write it. And I will become their God, and they will become my people.”
34 “And they will no longer teach each one his neighbor and each one his brother, saying, ‘Know Jehovah!’ for they will all know me, from the least to the greatest of them,” declares Jehovah. “For I will forgive their error, and I will no longer remember their sin.”
The GB, and religious leaders in general, do not accept that people can know God on their own. Yet, that's exactly what God said would happen to people who are part of this new covenant. Thus, the myriads of books, magazines, etc. that WTBTS produces to 'teach people about Jehovah' tells me that JWs in general are not part of this new covenant because they would not need all this stuff if they were.